A lab-made screen-printed electrode as a platform to study the effect of the size and functionalization of carbon nanotubes on the voltammetric determination of caffeic acid

This study describes the fabrication of carbon screen-printed electrodes (SPEs) from alternative and low-cost materials. The carbon conductive ink was prepared from a mixture of graphite powder and colorless nail polish. To produce the SPEs, the ink was spread on polyester overhead projector sheets, which acted as a flexible and water-impermeable substrate. To improve the analytical performance, the SPEs were chemically modified with multiwalled carbon nanotubes (MWCNTs) by a simple drop-casting procedure. The effects of the acidic functionalization and size of the MWCNTs on the electrochemical behavior of the SPEs were evaluated. As a proof of concept, the MWCNT/SPEs were applied to determine caffeic acid (CA) in tea samples. CA is a biologically important molecule with antioxidant activity and several beneficial effects on human health, therefore its determination in food and beverages is relevant. The SPEs modified with the acidic-functionalized longer diameter MWCNTs presented the broadest linear range for CA, from 2.0 mu mol L-1 to 50 mu mol L-1. These MWCNTs also provided low limits of detection and quantification, 0.20 and 0.66 mu mol L-1, respectively. Tea samples were analyzed by using these electrodes and recovery percentages from 99 to 109% were obtained with low relative standard deviation values, demonstrating the accuracy of the proposed sensor. Moreover, the developed SPE allowed performing the voltammetric measurements using only 100 mu L of solution which is extremely attractive from an economical and environmental point of view. Therefore, this study brings exciting advances in the electroanalysis field regarding the importance of the development of disposable electrochemical sensors with improved performance and capability to perform electrochemical measurements in microvolumes.